1. Field of the Invention
The invention relates to a small integrated antenna system for satellite communications. More particularly to low profile omni-directional satellite antennas having a compact height and relatively good VSWR immunity to adjacent grounding structures. The invention is particularly addressed to use in land-mobile position and locating systems.
The invention includes several novel features. These include a low loss refracting dome enclosing a top fed, dual bi-filar helix in the form of a conical frustum, having resonant arms shorted to a shielding ground plane. The helix is driven by a unbalanced to balanced feed network including a low loss shielded-suspended-substrate balun/splitter stripline-like circuit combined with the ground plane. The balun/splitter includes compensating balun arm lengths to achieve a uniform azimuthal radiation pattern. An efficient, level controlled, grounded base, Class C, power amplifier using an emitter bias current to control the base-emitter conduction threshold. The emitter current bias input is controlled from a directional coupler sampling the transmitted forward power.
The electronics are integrated directly into the antennas ground plane structure and shielded from the radiating helix to form very compact and efficient antenna system. This provides a structure which meets stringent radiation pattern requirements for INMARSAT satellite communications. The combination of the helix frustum shape and refracting dome provide a uniform radiation pattern in elevation. The conical structure with integral ground plane provides a system having reduced height and reduced VSWR sensitivity to the effects of mounting on vehicle rooftops.
2. Background of the Invention
In long-haul shipping, speed, timing and punctuality are critical factors for successful companies. Delivering cargo exactly where it needs to go, on time, requires the ability to communicate with every vehicle in a fleet, at all hours of the day, anywhere in the world. Mobile satellite communication systems have been combined with satellite position locating systems to use in marine shipping and in long-haul trucking. Long haul trucking in particular requires mobile communication/position locating units having light weight, low profile antenna systems.